The subject says it already. I have an IMAP folder called "Spam". I use this folder to store spam messages which are not detected as spam by SpamAssassin. Sometimes when I look in the folder after moving a spam message into it I see a message there as well which is not spam and which I haven't moved into that folder. When I refresh the message is disappeared.
I think it might be a caching problem. I turned caching off to see if it solved the problem. I post my findings in this thread.
PS: just discovered RoundCube yesterday, and really like it!
I think (after a couple of days of testing) that the issue I had was due to the caching mechanism. Works better now. I hope the caching will be improved in future releases.